The invention relates generally to polarimetric synthetic aperture radar (SAR). In particular, this invention enables accurate detection and monitoring of objects with specific sizes and shapes across a wide area imaged by a polarimetric SAR platform.
SAR is a coherent, microwave imaging radar system with day, night and all-weather capabilities. SAR systems provide high resolution imagery over wide areas under conditions of poor visibility, which make them useful in target detection, tracking and monitoring applications. SAR systems can collect data in three modes: strip-map mode (i.e., antenna pointed in a fixed direction relative to the flight path), spot-light mode (i.e., antenna maneuvered during data collection to radiate only a desired area of terrain), or scan mode (i.e., antenna maneuvered during data collection to radiate a desired swath at any arbitrary angle relative to the flight path). Also, SAR systems can be used in the interferometric mode to obtain high resolution digital terrain elevation information over the imaged area. SAR systems can be installed on airborne and/or satellite platforms.
A polarimetric SAR system transmits and receives pulses with both horizontal & vertical polarizations. Polarimetric SAR imagery consists of two, three or four independent channels of complex data (amplitude plus phase) consisting of HH (Horizontal transmit, Horizontal receive), HV (Horizontal transmit, Vertical receive), VV (Vertical transmit, Vertical receive), and VH (Vertical transmit, Horizontal receive). For a fully polarimetric or quad-polarization SAR system (four channels), all four combinations HH, HV, VV and VH are employed. In the case of three channels, either VH or HV is dropped. Two channel systems exist where there is one transmit polarization and dual receive channels (HH and HV combination or VV and VH combination). The complex, 2×2 polarimetric scattering matrix of each individual source of radiation scatter (or “scatterer”) is represented by the returns from all four channels HH, HV, VH, VV. The scatterer response is reciprocal if and only if the condition HV=VH holds.
One conventional method for utilizing the extra information in polarimetric SAR produces a false color image in which each pixel has a red-green-blue (rgb) value based on the HH, HV, and VV amplitudes at that pixel. This method ignores the phase information contained in the polarimetric SAR data and has some limited utility in classifying ground cover over large areas. The typical approach to exploiting the polarimetric SAR data applies a decomposition transformation to the three or four complex (i.e., real plus imaginary) numbers associated with each pixel in the complex SAR image(s). Standard decompositions include Huynen, Freeman-Durdin, Cameron, Touzi, Krograger and Cloude, used by the technical community with proponents for each. These apply a non-linear transformation to the complex numbers associated with a pixel and classifying the result as one of a small number of basic scattering center types. For example, the Cameron decomposition classifies all symmetric scatterers (i.e., scatterers that have an axis of symmetry in the plane orthogonal to the radar's line of sight) as either trihedral, dihedral, cylinder, narrow diplane, dipole; or quarter wave.
For all of the standard decomposition approaches, each pixel gets classified as one of the scattering types defined by the decomposition used. However, no conventional mechanism is available to determine the validity of the classification. In most cases, there are multiple scattering centers in the resolution cell (e.g., pixel), and the classification is performed based on the combined return from the multiple scattering centers irrespective of the actual shape of any of the scattering centers in the resolution cell/pixel. This in turn leads to an unacceptably high false alarm rate when trying to detect any specific objects.
Independent of polarimetric SAR, a well known technique is to convert a full resolution SAR image into a sequence of lower resolution sub-aperture images that can be displayed as a short movie loop. This sequence looping has provides a visual cue to stationary scattering centers as well as speckle phenomena and non-stationary scattering centers. This has significantly aided manual analysis of single polarization SAR images, but has not resulted in any significant automated analysis methods to date. Sub-apertures can be formed in either the “fast time” (swath-range) or “slow time” (cross-range, azimuth) dimensions. Another well known technique incoherently averages sub-aperture images (for a single polarization) to produce a “multi-look” image having reduced speckle (i.e., the graininess associated with SAR imagery).